Toi debuted with the Tigers (then called the Osaka Tigers) in 1958, going 4 for 10 with two doubles. He made his pro debut batting for Kazuomi Nakamura. His first hit was off Noboru Akiyama. He hit .248/.271/.360 in 129 plate appearances over 75 games in 1959, backing up Katsumi Fujimoto at 1B and pinch-hitting regularly. Late in the year, he took Akiyama deep for his first career home run. He batted .272/.350/.441 in 91 games in 1960; his six triples tied Noboru Inoue and Tatsuro Hiroka for 9th in the Central League. He hit .237/.310/.331 in 155 plate appearances over 83 games in 1961 and .264/.319/.355 in 119 plate appearances and 73 games in 1962. He was 0 for 7 in the 1962 Japan Series as Hanshin fell to the Toei Flyers.

Finally replacing Fujimoto as the regular at first, he hit .284/.371/.441 in 1963 and .282/.333/.425 in 1964. In the 1964 Japan Series, his last Japan Series, he went 5 for 24 with a walk, double, run, two RBI and 8 strikeouts as Hanshin lost out to the Nankai Hawks. In 1965, the Yamaguchi native smacked 16 home runs and produced at a .256/.307/.479 clip. He tied Takeshi Kuwata for 8th in the CL in doubles (21), tied Shigeo Nagashima for 3rd in triples (5) and was 10th in home runs. He hit .326/.390/.475 with 27 doubles in 1966. He made his first CL All-Star team and was among the league leaders in average (2nd, .018 behind Nagashima), OBP (5th, between Kazuyoshi Yamamoto and Toshio Naka), slugging (7th or 8th, depending what the cutoff was), OPS (6th, between Yamamoto and Morimichi Takagi), hits (141, 4th), doubles (3rd behind Makoto Matsubara and Nagashima), RBI (tied for 10th with Sadayuki Tokutake) and walks (48, 8th, between Kazuhiko Kondo and Tokutake).

Goro produced at a .309/.366/.400 clip with 28 doubles in 1967 and again was an All-Star. He was 5th in the CL in average (between Yamamoto and Shiro Takegami), 6th in OBP (between Isao Shibata and Shinichi Eto), 2nd in hits (151, 3 behind Taira Fujita), tied with Matsubara for second in doubles (two shy of Fujita), 7th in walks (45, between Yamamoto and Kazuhiro Yamauchi) and tied with Shibata and Kosuke Matsuoka for 6th in strikeouts (70). He slumped to .262/.309/.398 with 26 doubles, 13 home runs and 68 RBI in 1968, placing among the leaders in hits (133, 10th), doubles (6th), RBI (9th, between Yamauchi and Yamamoto) and strikeouts (86, 4th, between Lou Jackson and Toshio Kato).

In 1971, Toi slumped to .224/.294/.317 with six home runs. He did get his 100th career homer, taking Hisanobu Mizutani deep July 2. He hit .262/.313/.426 in 1972 and .296/.351/.390 in 1973. Had he qualified, he would have been third in average behind Oh and Tsutomu Wakamatsu; he was within 20 plate appearances of qualifying. He batted .260/.316/.408 with 14 dingers in 1974. He went back to the bench in 1975, backing up George Altman; he hit .248/.331/.333 in 149 plate appearances over 80 games, again pinch-hitting regularly as he did early in his career. In 1976, he went 9 for 43 with 5 walks, 3 home runs and 12 RBI, backing up Hal Breeden at 1B and pinch-hitting the vast majority of his time. He hit .255/.304/.392 in 56 plate appearances in 1977, never taking the field. He did hit the 3,000th homer in Hanshin franchise history, going deep April 24.

Overall, Toi had batted .272/.332/.409 in 1,919 NPB games, with 475 runs, 253 doubles, 137 homers, 688 RBI and 471 walks. His 96 pinch-hit RBI?hits? were the all-time team record; the mark was later broken by Hiroshi Yagi and Shinjiro Hiyama. Through 2011, he was among the career NPB leaders in games played (54th, between Kenjiro Nomura and Mitsuo Motoi), doubles (98th), intentional walks (48, tied for 66th with Nomura) and strikeouts (tied for 90th with Kinji Shimatani).

He later was batting coach for the Tigers (1978), was a baseball commentator on radio and ran a pub. He died of lung cancer.